1. Field of the Invention
The present invention relates to an optical package substrate and an optical device, and more particularly to a package substrate having a surface configuration which is suitable for mounting optical components and/or optical elements thereon, a method for molding such a package substrate, and an optical device which is constructed by employing the package substrate.
2. Description of the Background Art
Optical communication systems employing optical fibers are evolving from conventional long haul communication systems into subscriber communication systems. Subscriber-type optical communication systems require the use of small and inexpensive optical devices.
In conventional optical devices, optical components such as optical fibers and/or lenses and optical elements such as lasers and/or photodiodes are deployed in a coaxial arrangement. Usually, the positioning of the optical fibers and lenses requires a high precision, e.g., a tolerance of ±1 μm. Therefore, a so-called active alignment technique has been used for the assembly of an optical device, where the positioning of the components is adjusted while driving the optical elements with laser light actually being led therethrough. However, this technique requires complicated tasks, and is time-consuming, thus presenting substantial cost problem.
On the other hand, a so-called passive alignment, in which the components are positioned without the aforementioned adjustment, has attracted attention as a technique for simplifying the assembly of an optical device. Typical examples of this technique are used in optical devices such that light that is guided through optical fibers and optical cavities is deflected by 90° so as to be received by a photodiode of a surface reception type. Various ideas have been proposed for this technique, in documents such as Japanese Patent Laid-Open Publication No. 11-326662 and Japanese Patent No. 2687859.
FIG. 19 is a cross-sectional view showing an exemplary optical device structure which utilizes a conventionally-proposed deflection method.
As shown in FIG. 19, an end face of an optical fiber (or an optical waveguide) 191 is ground (angled) at 45°, with a reflection mirror 192 being provided on this end face. Light that is guided through the optical fiber (optical waveguide) 191 which is incident from the left direction in FIG. 19 has its optical path turned (i.e., deflected) by 90° at the mirror 192 so as to be received by the photodiode 193. The photodiode 193 may be of a surface reception type as that shown in FIG. 19, or of a different type called a “waveguide type”.
A surface reception type photodiode has a large area for receiving light, so that it requires a positioning precision of only about ± tens of microns (μm) with respect to an optical fiber or an optical waveguide. Therefore, the mounting of a surface reception type photodiode can be realized through a passive alignment, which involves forming markers on the substrate as reference points for the mounting process. However, when light which is guided through the optical fiber or optical waveguide needs to be received directly at the front face of the surface reception type photodiode, it is necessary to mount the surface reception type photodiode in an upright position on the substrate, which makes mass production more difficult. An example of this situation is the monitoring of a laser output, where surface reception type photodiodes are a common choice.
On the other hand, a waveguide type photodiode has a light receiving layer on the order of several microns. Therefore, a positioning precision as stringent as ±1 μm is required in order to ensure that light guided through an optical fiber or optical waveguide is properly coupled to a waveguide type photodiode. Thus, active alignment is usually employed for waveguide type photodiodes, thereby presenting a substantial mounting cost problem.
Thus, it can be seen that the construction shown in FIG. 19, where light which exits from an optical fiber or optical waveguide is deflected by 90°, can be highly effective in terms of mass production and mounting costs, because it makes it possible to mount a surface reception type photodiode in a face-down manner through passive alignment.
FIG. 20 shows another exemplary optical device structure utilizing the conventional deflection method described in Japanese Patent Laid-Open Publication No. 11-326662.
Referring to FIG. 20, this optical device features a reflective member 201, which is formed by processing a portion of an optical waveguide into a reflective surface for reflecting light. Light which is propagated through the waveguide layer 202 to exit at its end face is deflected by 90° so as to be received by a surface reception type photodiode 203.
As illustrated by the above two examples, conventional optical device structures are based on the concept of deflecting an optical path of light exiting from an optical fiber or optical waveguide so as to allow the exiting light to be received by a surface reception type photodiode, which requires a relatively low receiving positioning precision, with a view towards reducing the device cost.
However, the conventional optical device structures illustrated in FIGS. 19 and 20 require an additional process of working an end face of an optical fiber (or an optical waveguide) into a face having a 45° slope. Moreover, in order to minimize the scattering loss at the processed surface, it is essential to secure a high planar precision, and as a result, processes such as grinding, dry etching, wet etching and cutting must be carried out with a good reproducibility. Especially in the case where an optical fiber processed so as to have a slanting end face is employed, light exiting from the optical fiber may not be properly incident on a photodiode if the optical fiber fails to be mounted in the correct orientation, due to rotation or the like.
Thus, while conventional optical device structures provide advantages that are associated with simplified photodiode mounting, they also require additional processing steps, which detract from the cost advantages.